1. Field of the Invention
This invention relates to the field of synthetic resins containing modified natural resins and, more particularly, to blends of ethylene-vinyl ester copolymer and dibasic acid-modified rosin ester.
2. Description of the Prior Art
Numerous adhesive compositions, largely of the hot melt variety, containing an ethylene-vinyl ester copolymer and a resin and/or rosin-based tackifying agent are known. Illustrative of such compositions are the adhesive materials disclosed in U.S. Pat. Nos. 2,485,248; 3,232,895; 3,484,405; 3,657,171; 3,849,353; 4,140,733; 4,167,433; 4,168,253; and 4,183,384. The compositions described in the last mentioned patent, U.S. Pat. No. 4,183,384, are fairly representative of the prior art adhesives and are prepared from a latex such as an ethylene-vinyl acetate latex and, as a tackifying agent, an aqueous dispersion of a resinous polyelectrolyte prepared from the reaction product of a rosin, a monoalcohol and a carboxylic acid, e.g., a dibasic unsaturated acid.
Ethylene-vinyl ester copolymers employed in known hot melt adhesive systems typically possess a vinyl ester content, usually vinyl acetate, of from about 10 to 30% by weight. Copolymers possessing this range of vinyl ester content are unsuitable for blending with rosin esters in many common solvent systems due to their relatively poor solubility. The use of ethylene-vinyl ester copolymers in the 40 to 45% by weight vinyl ester range (which are soluble in a variety of organic solvents) as modifiers for nitrocellulose and chlorinated rubber is well known. However, at this level of vinyl ester, such copolymers have been found to be incompatible with dibasic acid-modified rosin esters in any useful proportions. While incompatability is unimportant insofar as the hot melt adhesives are concerned since appearance is not a significant consideration with these products, coating materials must possess an acceptable degree of clarity to be regarded as commercially useful.